For wider application of robot manipulators to assembly tasks, establishment of the force control technology is fundamental. The objective of this research has been to develop a prototype force-controlled robot system for assembly tasks, to establish a force control algorithm, to obtain an assembly planning method based on analysis of assembly tasks and utilization of visual feedback.1. Design and Fablication of a Force-Controlled Robot System for Assembly Tasks. This prototype robot has the following feature. (1) The arm portion consists of three orthogonal prismatic joints, each joint having a direct-drive linear motor with low friction. This provides good force control performance and high structural rigidity. (2) The wrist portion has a compact structure with three rotational direct-drive joints.2. Establishiment of Force Control Algorithm. The dynamic hybrid control approach proposed by us before has been extended to the case of unknown objects and to the case of multiple robots. A new force control algorithm that changes the control mode between posiltion control and force control smoothly by using the impedance control scheme has been proposed.3. Analysis of Assembly Tasks and Establishment of Assembly Task Planning Method. For autonomous planning of assembly tasks, a method has been proposed based on the contact state transition network. By specifying a cost function based on the difficulty of state transitions, the method finds an optimal sequence of the contact states on the network. A method to determine a time-trajectory that links this sequence of contact states is also biven.4. Verification by Experiments. Using the robot system developed in 1, the effectiveness of the force control algorithm proposed in 2 has been shown by several experiments.